Instead of an elevated sleeping platform, I'm looking into making a gear SHELF above the sleeping floor.

Could I for example run plywood on supports across, resting on those elevated mouldings (yellow dots) - can those side pieces support weight, or are they hollow and/or risk damage to whatever is under/behind it?

Similarly, could I "build up" a raised support from the blue dots? Similarly, with the support beam and weight on it, is this fine or would it punch through whatever is there?

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So long the plywood has supports towards the middle of the cargo area, then you'll be okay. The yellow dot areas CAN support weight, but only if the weight is evenly distributed.

If you plan on using plywood to rest only on those yellow dot areas (without supports), then you'll definitely have issues with the plywood bowing and/or causing damage

As for the blue dot areas, it is also fine to have support beams without pushing through the panel.

There are plenty of folks who have the idea you've mentioned. I assume your intent is to have front support beams on the blue areas only, and no beams in the rear (just resting on the wheel wells). If that's the case, the amount of weight that you intend to put on this shelf will be the biggest hinderance as the plywood will bow.

Hmm, good point about some type of middle support. I suppose I could add one, though it would "Separate" the left+Right side when sleeping (maybe a good thing! ha!) but as a result also makes dealing with a mattress more difficult - can't just be one larger inflatable mattress...

I guess another option is to actually provide support via the roof, but I imagine those anchors are even less ideal for supporting decent weight?

Obviously tons of people with diff cars with sleeping platforms and drawer systems, but I'm not finding nearly as much options for base sleeping + shelf above. Maybe there is a reason...

How much weight are you intending to put on this shelf? If it's just bags and such (total weight under 100lbs), I'm sure you'll be able to get away with it, just use two sheets of 1/2" or 3/4" plywood to limit the bowing.

Also, how high will this shelf be? Will it be resting directly on the wheel wells? If so, that only provides like 6-8 inches between the bottom of the shelf and cargo tray which is not a lot of space if you'll be using a sleeping mattress. FWIW, I use an Exped Megamat and it's about 4 inches thick.

It all depends what you're putting on the shelf because size/weight will be the primary limiting factors, especially if you're trying to sleep under it.

I'd likely increase height above the wheel wells... yeah, need to take some specific measurements for an eventual setup. I could see weight being under 100lbs, but certainly/easily over 50.